In recent years, fiber structures, such as nonwoven fabric and paper, are increasingly used in which the constituent fibers are adhered using heat-fusible fibers. Such fiber structures include interior material for cars, base fabric for carpets, highly efficient filters, supports for water processing membranes, hook-and-loop fasteners, and others. Furthermore, the demand of the thermoplastic composite is also greatly extended in some fields, such as a structure component for an aircraft, and a housing of home electronics. The thermoplastic composite is produced by preparing a paper or nonwoven fabric comprising inorganic fibers, such as carbon fibers and glass fibers, as a reinforcement fiber and heat-fusible fibers; and processing them at a temperature high enough to fuse the heat-fusible fibers so as to make both fibers unified like a matrix.
The heat-fusible fibers include various types of fibers, such as a polyethylene-polypropylene composite fiber which comprises polyethylene as an adhesive component; a nylon-polypropylene composite fiber in which a copolymerized nylon is used as an adhesive component; a composite fiber of polyethylene terephthalate and ethylene vinyl alcohol copolymer in which the ethylene vinyl alcohol copolymer is used as an adhesive component; a composite fiber of amorphous polyester and polyethylene terephthalate; and others. In particular, polyester type heat-fusible composite fibers are widely used in various fields because of their outstanding light resistance, mechanical property, durability, and further recyclability.
Major polyester (hereinafter sometimes abbreviated as PES) type heat-fusible composite fibers widely proposed are so-called sheath core composite fibers arranging as a core part a crystalline PES type polymer represented by polyethylene terephthalate and as a sheath part an amorphous PES type polymer comprising a component disturbing crystallinity of the polymer as a copolymerizable component, such as isophthalic acid (for example, see Patent Document 1). Since these heat-fusible fibers consist of a crystalline core part having a high melting point, and an amorphous sheath part being softened at a lower temperature than the melting point, they can be used as an adhesive component by heat-treating them at a temperature higher than the glass transition temperature of the sheath part so as to allow only the sheath part to be fused. These heat-fusible fibers can keep their fiber form without fusing the core part. In order to meet the demand for adhesion temperature to be applied or the heat-resistant demand for the real use, many trials have been widely proposed in which the copolymerization ratio, etc. are designed in amorphous PES type polymers.
On the other hand, a sheath core composite fiber has been also widely proposed which arranges in the sheath part an amorphous PES type polymer comprising a special monomer as a copolymerizable component in order to improve heat resistance (for example, see Patent Document 2).
There also has been widely proposed a resin composition or a fiber which has a glass transition temperature higher than the glass transition temperature of a PES fiber and comprises cellulose, polycarbonate, polysulfone, polyarylate, polyether sulfone, etc as a blendable component (for example, Patent Document 3).
For the purpose of enhancing the glass transition temperature of polyethylene terephthalate fiber consisting of polyethylene terephthalate, the fiber or film which includes polyetherimide (hereinafter sometimes abbreviated as PEI) type polymer as a blendable component which shows good compatibility with polyethylene terephthalate (for example, see Patent Documents 4 and 5).